[mythtv-users] Why RAID 5 stops working in 2009

[John Drescher]

>>> Also even in this unlikely case with mdadm you should be able to copy
>>> the bad drives to new drives and force the arrays to sync even with
>>> the missing data. I am not sure if hardware controllers allow you to
>>> sync disks that have problems like this.
>>
>> If that sector is a parity sector, and its lost its original data, a restore
>> is going to destroy a lot of data from what I've heard.
>
> You'll lose one stripe of data.  Or more accurately, one stripe will be
> silently corrupted.  Whether this is a big deal or not depends on where
> that stripe is.  If it's in the middle of an MPEG video file it's
> probably no big deal.  If it's a database table, or a subdirectory, you
> better have a backup.
>
Agreed.

BTW the complete scenario got cut off the discussion so I would like
to ask about that. The question was in raid6 if one drive was dead and
two other drives had unreadable sectors. And then I said that even
though this was highly unlikely I believed mdadm would be able recover
if you forced it.

Now I still believe it will recover this stripe as long as only 1
drive in the stripe has bad sectors. Is this correct? Or will mdadm
not know that the stripe data for the drive with the bad sector was
incorrect?

John